Save it (Ctrl+X).
At this point you can either remove your modem and re-insert it for it to be switched to modem mode automatically or use the following line to do it manually:

usb_modeswitch -c /etc/usb_modeswitch.d/12d1:1c0b

Check if the modem is switched

lsusb | grep Huawei

The output should be something like this:

Bus 003 Device 003: ID 12d1:1c05 Huawei Technologies Co., Ltd.

Note that the product ID has changed from 1c0b to 1c05. It means that the USB stick can now be used as a modem. Also note that it can take some time (probably not more than 15 seconds) for the modem to switch if you removed and re-inserted it instead of running usb_modeswitch manually.

Connecting

The easiest way to connect is with sakis3g.
Install ppp (base) and net-tools (core) if not installed:

Save it (Ctrl+X) and now you are almost finished. What you need to do is to run sakis3g connect at system startup. For some reason putting it in /etc/rc.local does not work so a good option if you are using X but not using a login manager like GDM is to put it in your .xinitrc:

sudo sakis3g connect &
exec openbox

For this you first need to edit /etc/sudoers and add the following line: